Flagstone calculator

Flagstone is sold by the ton, and how far a ton goes depends mostly on how thick the stone is: suppliers quote roughly 100–130 sq ft per ton for 1–1½ in stone and 80–110 sq ft for 1½–2 in. The gravel base and sand setting bed are what go under a dry-laid patio.

How much flagstone per ton?

Flagstone is split stone — sandstone, limestone, bluestone, slate — and it is sold by weight, so the question is how many square feet a ton covers. That depends mostly on thickness. A sandstone flag weighs about 140 lb per cubic foot of solid stone (one supplier’s figure), so a perfectly even 1¼ in slab would cover about 137 sq ft per ton; real flags are uneven and thicker in places, which is why suppliers quote a little less.

tons of flagstone = area × (1 + waste) ÷ coverage per ton
pallets = tons ÷ tons per pallet
base (tons) = area × base depth × 1.25 (compaction) × 1.5 tons per loose cu yd × 1.05

Worked through for the defaults: a 12 × 12 ft patio is 144 sq ft; with 10% for cutting and breakage that is 158 sq ft, and at about 120 sq ft per ton you need about 1.32 tons — one pallet if the yard packs 1½ tons to a pallet. Under it go about 3.5 tons of crusher run for a 4 in compacted base and about 0.8 tons of sand for a 1 in setting bed.

What goes under a flagstone patio

A dry-laid patio — stones on a bed, joints filled with sand, stone dust or plants — needs the same thing a paver patio needs: firm, well-drained ground. A common build is about 4 in of compacted crusher run with 1–2 in of coarse sand or stone dust on top to bed each stone, more base on clay or where frost heaves. Stepping stones in a lawn often go straight into the soil. Thin flags are normally mortared onto a concrete slab instead, because they crack when laid loose. These are typical approaches, not a standard; your stone yard can tell you what works locally.

Buying flagstone

• Stone comes on pallets, commonly around 1½–2 tons each; set Stone per pallet under More options to the yard’s figure.
• Pick your pallets at the yard if you can. Big, even flags go further and lay faster than a pallet of small pieces.
• Buy it all at once. Colour and thickness vary from one quarry lot to the next.
• Allow for waste: broken pieces, cuts along edges and the odd stone you won’t use. 10% is a common allowance; more if you want tight joints.

For the gravel under it, see gravel types for driveways and paths and how to lay a paver patio — the base work is much the same.